Preparation and growth mechanism of nickel nanowires under applied magnetic field

1National Key Laboratory of Nano/Micro Fabrication Technology, Key Laboratory for Thin Film and Microfabrication of the Ministry of Education, Institute of Micro and Nano Science and Technology, Shanghai Jiao Tong University, Shanghai 200240, China 2School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China 3Zhongyuan University of Technology, Zhengzhou City, Henan Province 450007, China *Corresponding author. E-mail: yfzhang@sjtu.edu.cn; Tel.: +86(0)21-34205665; fax: +86(0)21-34205665 Preparation and growth mechanism of nickel nanowires under applied magnetic field

Morphological control in nanostructures has become a key issue in the preparation of electronic, photonic devices as well as functional materials [1,2]. In addition, much attention has been focused on one-dimensional (1-D) nanostructure such as nanorods, nanowires, nanofibers and nanochains due to their potential applications in nanodevices [3][4][5]. As a typical magnetic material, 1-D metallic Ni nanomaterial in uniform morphology and high purity has become increasingly mandatory for specific applications such as microwave absorbing materials, magnetic recording media, gas sensors, drug deliveries, commercial batteries and catalysts [6][7][8][9][10][11][12]. To date, much effort has been made to prepare Ni nanomaterial with 1-D structure. Template methods are the most common synthesis technique. For example, Yu et al. prepared nickel nanowire arrays in alumina templates by using a chemical electrodeposition method [13]. Zhang et al. fabricated nickel hollow fiber using natural silk as the template [14]. It is well known that template-based methods are not facile and in fact, it is difficult to synthesize a large amount of nanomaterials using the template-based route [15]. In comparison, chemical reduction approach is much simpler and having lower cost.
Recently Niu et al. fabricated acicular nickel nanocrystallites with an average length of 10 μm and a diameter of about 200 nm, using PEG and CTAB surfactant in a sealed autoclave under an external magnetic field, under high temperature and pressure [16]. Although Gong et al. synthesized Ni nanowires under normal pressure, ethylene glycol was used as the solvent [17]. In recent years, our research group has reported the facile synthesis of Ni nanowires in aqueous ethanol solution [18,19]. This is a much more promising approach to synthesize magnetic nanowires in large scale since it is simple, low-cost, high yield, and environmentally friendly.
In previous work, we have reported the synthesis of Ni nanowires [18,19]. In this investigation, we studied the

Results and Discussion
The role of the hydrazine hydrate as a reducing agent in the reaction was investigated. It was found that no grey-black products formed if hydrazine hydrate was absent even at high temperature of 80 o C and strong magnetic field of 0.5 T. This indicates that hydrazine hydrate played a very important role in the reaction as a reducing agent. Moreover, it was found that the reaction efficiency strongly depended on the pH value of the solution. When pH value was 13.7, the reaction will complete within 30 min at 50 o C. However, when the pH value was 13.0, no nanowires were produced until the reaction temperature was increased to 86 o C. If the pH value was below 13.0, the reaction would not take place even at 100 o C. Thus, choosing the proper pH value is essential on the synthesis of the nickel nanowires as described in this investigation. The important role of pH value on formation of Ni nanomaterials has also been discussed by other groups [20,21]. In addition, we found that higher reaction temperatures will accelerate the reaction processes, that is, it will need less time to complete the reaction at a higher temperature. At the selected reaction temperature of 50 o C at pH value of 13.7, the reaction time will be 30 min.  No other peaks were observed, implying that the nanowires prepared by this method have high purity and they are not oxidized. The lack of oxidation can be attributed to the fact that nitrogen gas was generated during the reaction process. This phenomenon was also observed and discussed by other researchers [18].
Since external magnetic fields played a very important role in formation of 1-D magnetic Ni nanowires [19,21], we investigated the products prepared without applying magnetic fields. Figure 4 shows the SEM images of the products at 50 o C for 30 min without magnetic field. One can notice that the products are nanoparticles and without any 1-D nanomaterials present (see Fig. 4a). From the magnified image shown in Fig.   4b, the size of the particles can be estimated to have a diameter of 100 nm. Therefore, it can be concluded that magnetic field is the prerequisite and mandatory condition to be present in order to grow the magnetic nanowires in this synthetic approach.
In order to illustrate the detailed growth mechanism of the nanowires and the role of magnetic field on the formation of the nanowires, we investigated three samples which were collected during the reaction process. Three samples were collected after the reaction took place for 10 min, 20 min and 30 min. Figure 5 shows that their SEM images and the corresponding schematic illustration of the growth processes of nickel nanowires. One can see at the beginning of the reaction, that is, during the first  reduce the reaction rate. This will improve the oriented growth of the nickel nuclei and form spherical particles [23]. In the second 10 min, the magnetic particles were subjected to magnetization in the magnetic field and aligned linearly by the magnetic force. The strong magnetic dipole interactions between magnetic particles will make them align tightly and rapidly, forming the magnetic particle-chains. At the same time, the formation of the chains would enhance the local magnetic field [24]. In the third 10 min, new nickel nuclei deposited near the chains as influenced by strong local magnetic field to form linear nanowires as show in Fig. 5c. The magnetic dipole interactions among the particles are so strong, it is difficult to destroy or break down the linear or chain-like structures even as they were subjected to an ultrasonic treatment for 2h. It must be noted that magnetic dipole interactions among magnetic particles can strongly affect the shape of the nanowires [25].